dt-bindings: iio: ad7124: Fix dtc warnings in example

With the conversion to DT schema, the examples are now compiled with
dtc. The ad7124 binding example has the following warning:

Documentation/devicetree/bindings/iio/adc/adi,ad7124.example.dts:19.11-21: \
Warning (reg_format): /example-0/adc@0:reg: property has invalid length (4 bytes) (#address-cells == 1, #size-cells == 1)

There's a default #size-cells and #address-cells values of 1 for
examples. For examples needing different values such as this one on a
SPI bus, they need to provide a SPI bus parent node.

Fixes: 26ae15e6 ("Convert AD7124 bindings documentation to YAML format.")

Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>